<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>

        div {
            width: 200px;
            height: 100px;
            background-color: purple;
        }
    </style>
</head>
<body>
<input type="button" value="改变样式" id="btn">
<br>
<br>
<div id="dv"></div>
<script src="common.js"></script>
<script>
    my$("btn").onclick = function () {
        my$("dv").style.width = "300px";
        my$("dv").style.height = "200px";
        my$("dv").style.backgroundColor = "pink"
    };
</script>
</body>
</html>